Geothermal resources of Réunion Island, Indian Ocean, studied by noble gas systematics in thermal springs

Abstract

La Réunion Island, home to the volcanoes Piton des Neiges and Piton de la Fournaise, hosts active hydrothermal systems fueled by a long-standing mantle plume. Understanding these systems is crucial for predicting their future potential as a geothermal resource. In this work, noble gases (Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), and Xenon (Xe)) were analyzed in 16 thermal springs to gain insights into fluid and heat sources as well as their pathways. Their helium isotopic composition or ³He/⁴He ratio is 13.9 ± 0.4 times higher than the atmospheric ratio (1.384 × 10⁻⁶). This ratio is comparable to those found in cumulates and xenoliths, associated with the Réunion mantle plume. Some samples show lower ³He/⁴He values than the plume source, indicating the addition of locally produced radiogenic ⁴He, likely from syenitic and basaltic products of Piton des Neiges and Piton de la Fournaise. The elemental (⁸⁴Kr/³⁶Ar and ¹³²Xe/³⁶Ar) and radiogenic noble gas ratios (⁴He/⁴⁰Ar*) appear to be influenced by magma degassing and, to a lesser extent, by boiling and recondensation of fluids in the shallower part of the hydrothermal circuit. This is supported by the heat/helium (Q/³He) ratio which indicates that mantle degassing mainly controls the transfer of heat and helium through convection at the interface between the magmatic source and the geothermal system. The distribution of helium isotopic composition of fluids suggests that the heat sources feeding the geothermal system reach maximum values in two zones: the cirque of Cilaos (a circular depression south of Piton des Neiges) along an N30° rift zone and the east of Réunion Island along the N120° rift zone between the two volcanoes. These zones are the most promising targets for future geothermal exploration.